West Nile virus is a flavivirus commonly found in Africa, West Asia, and the Middle East. It is closely related to the St. Louis encephalitis virus which is also found in the United States. The virus can infect humans, birds, mosquitoes, horses and some other mammals.

What are West Nile encephalitis, West Nile meningitis and “neuroinvasive disease” and West Nile fever?

The most severe type of disease due to a person being infected with West Nile virus is sometimes called “neuroinvasive disease” because it affects a person’s nervous system. Specific types of neuroinvasive disease include: West Nile encephalitis, West Nile meningitis or West Nile meningoencephalitis.

Encephalitis refers to an inflammation of the brain, meningitis is an inflammation of the membrane around the brain and the spinal cord, and meningoencephalitis refers to inflammation of the brain and the membrane surrounding it.

West Nile fever is another type of illness that can occur in people who become infected with the virus. It is characterized by fever, headache, tiredness, aches and sometimes rash. Although the illness can last as short as a few days, even healthy people have been sick for several weeks.

How long has West Nile virus been in the U.S.?

It is not known how long it has been in the U.S., but CDC scientists believe the virus has probably been in the eastern U.S. since the early summer of 1999, possibly longer.

Is there a vaccine available to protect humans from West Nile virus?

No. Currently there is no West Nile virus vaccine available for humans. Many scientists are working on this issue, and there is hope that a vaccine will become available in the next few years.

Should people take the West Nile virus vaccine that is licensed for use in horses?

No. This vaccine has not been studied in humans and could be harmful. The effectiveness of this vaccine in preventing West Nile virus infections in horses has yet to be fully evaluated, and its effectiveness in humans is completely unknown.

Veterinary vaccines are not manufactured with the same rigorous quality and purity standards required of human vaccines, nor are they required to undergo the extensive field testing required of human vaccines before they are licensed. For these reasons, veterinary vaccines and other veterinary drugs should never be used in humans.

How do people get infected with West Nile virus (WNV)?

The main route of human infection with West Nile virus is through the bite of an infected mosquito. Mosquitoes become infected when they feed on infected birds, which may circulate the virus in their blood for a few days.

The virus eventually gets into the mosquito's salivary glands. During later blood meals (when mosquitoes bite), the virus may be injected into humans and animals, where it can multiply and possibly cause illness.

Additional routes of human infection became apparent during the 2002 West Nile epidemic. It is important to note that these other methods of transmission represent a very small proportion of cases. Investigations have identified WNV transmission through transplanted organs and through blood transfusions.

If I live in an area where birds or mosquitoes with West Nile virus have been reported and a mosquito bites me, am I likely to get sick?

No. Even in areas where the virus is circulating, very few mosquitoes are infected with the virus. Even if the mosquito is infected, less than one percent of people who get bitten and become infected will get severely ill. The chances you will become severely ill from any one mosquito bite are extremely small.

Can you get West Nile encephalitis from another person?

No. West Nile encephalitis is not transmitted from person-to-person. For example, you cannot get West Nile virus from touching or kissing a person who has the disease, or from a health care worker who has treated someone with the disease.

Is a woman's pregnancy at risk if she gets infected with West Nile virus?

There is one documented case of transplacental (mother-to-child) transmission of WNV in a human. Although the newborn in this case was infected with WNV at birth and had severe medical problems, it is unknown whether the WNV infection itself caused these problems or whether they were coincidental. More research will be needed to improve our understanding of the relationship - if any - between WNV infection and adverse birth outcomes.

Nevertheless, pregnant women should take precautions to reduce their risk for WNV and other arboviral infections by avoiding mosquitoes, using protective clothing, and using repellents containing DEET (see Insect Repellent). When WNV transmission is occurring in an area, pregnant women who become ill should see their health care provider, and those whose illness is consistent with acute West Nile virus infection, should undergo appropriate diagnostic testing.

Besides mosquitoes, can you get West Nile virus directly from other insects or ticks?

Infected mosquitoes are the primary source for West Nile virus. Although ticks infected with West Nile virus have been found in Asia and Africa, their role in the transmission and maintenance of the virus is uncertain. However, there is no information to suggest that ticks played any role in the cases identified in the United States.

How many types of animals have been found to be infected with West Nile virus?

Although the vast majority of infections have been identified in birds, West Nile virus has been shown to infect horses, cats, bats, chipmunks, skunks, squirrels, and domestic rabbits.

Can you get West Nile virus directly from birds?

There is no evidence that a person can get the virus from handling live or dead infected birds. However, persons should avoid bare-handed contact when handling any dead animals and use gloves or double plastic bags to place the carcass in a garbage can.

Can you get infected with West Nile virus by caring for an infected horse?

West Nile virus is transmitted by infectious mosquitoes. There is no documented evidence of person-to-person or animal-to-person transmission of West Nile virus. Normal veterinary infection control precautions should be followed when caring for a horse suspected to have this or any viral infection.

Can you get WNV from eating game birds or animals that have been infected?

There is no evidence West Nile virus can be transmitted to humans through consuming infected birds or animals. In keeping with overall public health practice, and due to the risk of known food-borne pathogens, people should always follow procedures for fully cooking meat from either birds or mammals.

How does West Nile virus actually cause severe illness and death in humans?

Following transmission by an infected mosquito, West Nile virus multiplies in the person's blood system and crosses the blood-brain barrier to reach the brain. The virus interferes with normal central nervous system functioning and causes inflammation of brain tissue.

How long does the West Nile virus remain in a person’s body after they are infected?

There is no scientific evidence indicating that people can be chronically infected with West Nile virus. What remain in a person’s body for long periods of time are antibodies and “memory” white blood cells (T-lymphocytes) that the body produces to fight the virus. These antibodies and T-lymphocytes last for years, and may last for the rest of a person’s life. Antibodies are what many diagnostic tests look for when clinical laboratory testing is performed. Both antibodies and “memory” T-lymphocytes provide future protection from the virus.

If a person contracts West Nile virus, does that person develop a natural immunity to future infection by the virus?